The NHL doesn't have the authority to impose sentences in another league. It'd be great if the AHL would step up and carry suspensions down from the NHL, but that's something that only they can decide.
I'd love to see him sit and spin for 20 games at all levels...
EDIT -- Strike that, the AHL is reviewing it to either enforce, alter or dismiss the suspension. Link:
ESPN - Downie and out: Suspended winger sent to AHL - NHL